Update verify.sh script to point to bitcoin.org

Now that downloads are no longer hosted on Sourceforge, update the
script to retrieve the binaries and signature file from bitcoin.org.
Merge pull request #4441

ffebc1b Update verify.sh script to point to bitcoin.org (Michael Ford)
Make CBlockIndex a subclass of CBlockHeader
Unify CBlockHeader::hashPrevBlock and CDiskBlockIndex::hashPrev
Unify GetHash() and GetBlockHash()
Remove GetBlockHeader()
Use GetBlockTime() more